Expanded hollow polymeric microspheres based on thermoplastic polymers are known in the art and are used as low density fillers in various types of compositions such as coatings, adhesives including weld through adhesives, sealants including weld through sealants, and composites. Typically, the microspheres are prepared by emulsion polymerization of one or more monomers in the presence of one or more volatile substances such as a light, low boiling, hydrocarbon or halogenated organic compound. The monomers polymerize to form a shell that encapsulates the volatile substances which serve as blowing agents when the microspheres are expanded. The resulting microspheres are then heated to effect expansion of the shells as a result of the internal pressure created by the volatile substances together with a softening of the thermoplastic resulting from polymerization of the monomers. To help minimize agglomeration of the expanded microspheres and to provide such microspheres in a free-flowing form, it is known to coat the outer surfaces of the microspheres with processing aids such as calcium carbonate, silicate, or talc. See, for example, U.S. Pat. Nos. 4,722,943 and 5,180,752. In many applications, it is desirable for such microspheres to have as low a density as possible in order to reduce the weight and/or cost of the article prepared using the microspheres. However, it has been found that low density calcium carbonate-coated expanded microspheres can be flammable solids and thus may represent an explosion hazard. For example, microspheres having a composite density of 0.030 g/cm3 and containing 65 weight percent calcium carbonate as a coating based on the total weight of microspheres and calcium carbonate are flammable. When one increases the calcium carbonate coating to a level of 85 weight percent this reduces the flammability; however when added to a composition these same microspheres can significantly increase the flammability of the composition they are added to. Another process for dealing with the flammability issue is to include additional levels of flame retardants in the compositions that the microspheres are being added to. This is a less than satisfactory option for a number of reasons. The addition of the microspheres requires a large amount of flame retardant to be added to the composition and this increases the cost of the composition. In addition, the added flame retardant often increases the viscosity of the composition, which can be a substantial problem in processing of the composition and its subsequent final use. Since use of the microspheres themselves often adds to the viscosity of the composition, this additional viscosity from the flame retardant can be problematic.
It would therefore be advantageous to develop methods for preparing microspheres that don't add to the flammability of a composition they are added to. In addition, preferably the presence of the microspheres does not require additional flame retardant be added to the composition and thus not leading to changes to the viscosity of the composition caused by the flame retardant beyond the increase caused by use of the microspheres themselves.